Quality
of Two-Dimensional Images
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| Fig. 5 |
A single two-dimensional scan comprises a number of radial lines,
each displaying echoes in B- mode format. For accurate images both
the echoes shown on each B-mode line should accurately reflect the
structures encountered by the ultrasound beam, and the two-dimensional
image should contain the maximum possible number of lines. Because
the ultrasound beam is not a fine, laser-like line, objects that
lie off-axis are detected and generate artifactual multiple echoes
(Fig. 5). This
greatly impairs lateral resolution, the major limiting factor in
B-mode image accuracy. Reduction of beam width by focusing techniques
is therefore necessary to make the beam as narrow as possible. The
number of B-mode lines available to form a two- dimensional image
is determined by the velocity of ultrasound in the body (Fig.
6).
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| Fig. 6 |
For a 20 cm depth the maximum pulse rate is 3750 per second.
Also, the structures being imaged must not move significantly during
the period of the scan. A cardiac scan must therefore be completed
in 1/60th of a second or less depending on power supply characteristics
to 'freeze' valve motions, and a succession of such images made
for each cardiac cycle (1 second) to show the motion patterns. If
60 images are made each second, only 75 lines gives 2.5 lines per
degree, but if increased to 90 degrees, there is less than one line
per degree. Sector scanning systems must therefore compromise between
scans per second, scan angle, and line density.
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| Fig. 7 |
Fig.
7 summarizes two beams. With poor lateral resolution, multiple
images are formed of A and some echoes from B are also detected.
(2) Increasing line density without improving lateral resolution
simply gives more multiple images. (3) If resolution is improved
but line density remains low, a clear image is made of A but B disappears.
There must be both high line density and good resolution for high-quality
images of both objects.
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2004